Semiconductor gas sensors function by offering a change in electrical resistance in response to a shift in the local concentration of the gas of interest. In general the resistance of the sensor is some function of the concentration of the target gas. A major aim is to engineer a response, which is selective for a particular gas, without interference from other components of the atmosphere, including moisture (relative humidity).
Two general mechanisms are invoked to explain this concentration dependence on gas composition:
The mechanism operating at lower temperatures (generally within the range 200-500 C) involves reactions of the molecules of the target gas with chemisorbed species on the surface of the semiconductor, which is usually a metal oxide, and results in a change in near-surface charge carrier density. Examples of materials functioning through this mechanism can be employed in the detection and monitoring of either reducing gases (hydrogen, carbon monoxide, methane etc.) or oxidizing gases (nitrogen dioxide, chlorine, ozone etc.) in an air ambient. This response mechanism involves no change in the bulk composition of the semiconducting oxide.
Within a somewhat higher temperature range (approximately 500-700 C) a family of semiconducting oxides MOx, where M represents either a transition metal or a combination of metals, one of which is a transition metal, can be used for monitoring oxygen partial pressure. In this case the bulk stoichiometry does change, because the oxygen content of the material (the value of x) equilibrates with the prevailing oxygen partial pressure. The change in x is compensated by a change in the ratio of the valence states of the transition metal component of M, and once again the charge carrier density changes.
The carbon dioxide molecule does not engage in reactions equivalent to those of either the reducing gases or the oxidizing gases such as those equivalent to those of mentioned above, and heretofore no useful semiconductor sensor for carbon dioxide has been described.
Needs exist for new carbon dioxide sensors.